As an electrode catalyst, the ones having activated metal comprising platinum as a main component being supported on a carrier comprising a conductor such as carbon, tin oxide and the like have been used. Regarding various heterogeneous catalysts, the ones having activated metal comprising noble metal, non-noble metal, or alloys thereof as its main component being supported on a carrier comprising oxides such as alumina, titania, magnesia and the like have been used.
The performance of the electrode catalyst depends on the followings:
(1) particle diameter of activated metal,
(2) particle composition of activated metal,
(3) distribution of particle diameter (whether the particle diameter is uniform), and
(4) degree of dispersion on the carrier (whether the activated metal is dispersed uniformly on the carrier).
When the amount of the activated metal supported is the same, the larger the surface area of the activated metal, that is, the smaller the particle diameter and the higher the dispersion degree of the particles, the higher the performance of the activated catalyst. In addition, since platinum is high in cost, micronization of the activated metal, formulation of the activated metal into alloys, and uniform dispersion of the activated metal onto the carrier (supported with high dispersion degree) are required in order to suppress the amount of the activated metal being used.
As a manufacturing method for such electrode catalyst, a method involving the preparation of metal colloid solution by reducing chloroplatinic acid solution with a reducing agent comprising an organic acid, followed by allowing the resulting metal colloid to be supported on the carrier, has been suggested in Patent Literature 1 for example. In addition, as shown in Patent Literature 2, a method involving the preparation of fine particles by reduction using alcohol in the presence of an organic protecting agent, has been suggested.
On the other hand, besides suppressing the amount of platinum being used as mentioned above, usage of alloys as the catalyst has also been studied. Alloy catalyst is important also in terms of activating fuel electrode and air electrode. As the alloy catalyst, alloys of Pt and iron, cobalt, nickel, ruthenium and the like can be mentioned. Conventionally, when the alloy catalyst was manufactured, Pt particles already being supported on carbon was subjected to metal chloride solution by impregnation and the like. Subsequently, the resulting Pt particles were reduced at a high temperature of approximately 900° C. to give the alloy. Further, when the metal salt was to be supported on the carbon as the metal, alloy was manufactured by alcohol reduction. However, regarding the alloy catalyst obtained by such methods, the particle diameter of the catalyst particles was not uniform, and the composition thereof was also not uniform.
In addition, a study has been made to activate the catalyst with smaller amount of Pt. For example, Patent Literatures 3 and 4 disclose an electrode catalyst having a core-shell structure, where both of the core and the shell comprise a noble metal. The noble metal-containing particles used in Patent Literature 3 have a core-shell structure, the core portion comprising the noble metal alloy and the shell portion being formed on the outer periphery of the core portion and having a noble metal layer with a composition different from that of the core portion. In this manufacturing method, the noble metal containing particles are impregnated in strong acid such as concentrated sulfuric acid, and then the transition metal is allowed to elute, thereby increasing the content ratio of the noble metal at the surface of the particles. In addition, in another method, underpotential deposition of Cu is performed with a thickness of one atom layer on the surface of the core catalyst. Then, Cu is immersed in chloroplatinic acid solution to ionize Cu, thereby allowing Pt ion to go under substitution deposition as a zero-valent metal. In this process, a Pt coating having a thickness of approximately one atom layer can be obtained, however, such coating is imperfect, and thus the process need be repeated to form a shell. Therefore, the process becomes complicated and productivity is low.
However, in this method, it is difficult to deposit only the noble metal selectively, or to elute only the non-noble metal selectively so that only the noble metal is provided on the surface of the particles. In Patent Literature 4, thermal treatment is performed under reductive atmosphere, thereby obtaining a core-shell catalyst, the catalyst component composition of the shell comprising an alloy satisfying the relation of “noble metal≥non-noble metal”. However, in this method, non-noble metal exists on the outermost surface as the shell layer, and thus activity of catalyst is lowered since the non-noble metal melt under fuel cell operation. In addition, non-Patent Literature 1 discloses of manufacturing a core-shell particle having a Pt monolayer obtained by underpotential deposition. The catalyst obtained by this method uses gold, silver, palladium and the like as the core metal, and Pt is deposited on the surface shell. However, since Pt is a monolayer, stability is low, and thus elution of inner core metal cannot be suppressed sufficiently.
Regarding the operation of the fuel cell, in order to prevent the inner core metal component from eluting during its usage and to prevent the loss of catalyst activity, Patent Literature 5 suggests a manufacturing method of an electrode catalyst having catalyst particles covered with a several-atom layer of platinum skin layer supported on the carrier with high dispersion degree. This manufacturing method comprises the following three processes.
Specifically, the three processes are:
(1) a so-called “first reduction process” to prepare a nanocapsule solution by mixing two types of metal salts, solvent having a hydrophilic group, and a non-aqueous solvent; followed by addition of a non-aqueous solution having a reducing action; and then heating the resulting mixture to form an alloy particle in the nanocapsule,
(2) a so-called “second reduction process” to add a platinum skin precursor to the nanocapsule solution containing the alloy particles prepared in the afore-mentioned process (1), thereby allowing the platinum precursor being covered with the nanocapsule together with the alloy particles, followed by addition of a non-aqueous solution having reducing action to allow deposition of platinum skin layer on the surface of the alloy particles, and
(3) a so-called “platinum skin/alloy particles supporting process” to allow the platinum skin/alloy particles be supported on the carrier.